Singlet fission (SF) has recently attracted much interest for its potential to increase the theoretical maximum efficiency for solar cells. Many small molecule organic compounds have been found to undergo intermolecular and intramolecular singlet fission (xSF and iSF, respectively), the latter generally being considered more useful for implementation into devices since it alleviates the need for well-oriented, crystalline materials. However, polymeric systems are a rarity with only a few examples reported thus far21,57,65. Furthermore, benzodithiophene (BDT)-and thiophene dioxide (TDO)-based polymers were able to reach efficiencies of about 170% or greater, i.e., about ˜190%57 while other systems still lag behind.
While SF has been extensively studied in molecular crystals and aggregates, made possible by an intermolecular process, intramolecular SF (iSF) is not well understood in molecular and polymeric materials. Fundamentally, a core challenge is the development of materials capable of efficient intramolecular singlet fission. Challenges arise due to the many species that can form following initial photoexcitation, such as polarons and polaron pairs, creating difficulties in assigning iSF in polymers compared to small molecules. Recent work has gone into identifying the mechanism of iSF in small molecule systems, with pentacene dimers as a model system. It is unclear if charge transfer (CT) states play any role in the iSF process. Even with reports on iSF in polymers, there are much fewer design rules. Hence, there is a need to elucidate the iSF mechanism. It is an onerous task considering the lack of discrete chromophores, and that excited species in polymers are more delocalized.
Solar cells, also known as photovoltaic cells, are electrical devices that convert light energy directly into electricity by, what is known as, the photovoltaic effect. The photovoltaic effect is the creation of voltage or electric current in a material upon exposure to light. The photovoltaic effect is related to the photoelectric effect, although they are different processes.
Generally, when sunlight or any other light is incident upon a material, electrons present in the valence band can absorb energy and become excited from the absorption of energy. When the light energy exceeds bandgap, an electron can be promoted to what is referred to as the conduction band, which is the range of electron energies enough to free an electron from binding with its atom to move freely within the atomic lattice of the material as a delocalized electron, and become free. Then, as the highly excited, non-thermal electrons diffuse, some reach a junction where they are accelerated into a different material by a built-in potential (referred to as Galvani potential). The result is that an electromotive force can be generated. Thus, some of the light energy absorbed may be converted into useful electric energy. The photovoltaic effect can also occur when two photons are absorbed simultaneously in a process called two-photon photovoltaic effect.
Carrier multiplication refers to the phenomenon wherein absorption of a single photon leads to the excitation of multiple electrons from the valence band to the conduction band of a semiconducting material. In a conventional silicon solar cell, each photon is, in theory, only able to excite one electron across the band gap, and any photon energy in excess of the bandgap is dissipated as heat. In a material capable of carrier multiplication, high-energy photons excite on average more than one electron across the band gap, and so in principle the solar cell can produce more useful work.
However, silicon based solar cells are fundamentally limited in their production of useful energy. For example, if an incoming photon does not have sufficient energy, the cell will not absorb it. On the other hand, if a photon has too much energy, the excess energy is wasted as heat. In addition, it is believed that a silicon solar cell cannot generate more than one electron from a single photon absorbed. Thus, the conversion efficiency of photovoltaic cells by these combined effects, known as the Shockley-Queisser limit. The Shockley-Queisser limit is the fundamental upper limit to efficiency in single junction solar cells. This thermodynamic constraint limits the efficiency of single PN-junction solar cells to 33.7%, where a PN-junction is a boundary between two types of semiconductor materials. Scientists have spent decades looking for solutions to the problems posed.
Organic solar cell research has increased over the years and has seen the introduction of new materials, improved materials engineering, and more sophisticated device structures that provide increased power conversion efficiencies. Solar cells constructed of organic materials are becoming increasingly efficient due to the discovery of the bulk heterojunction concept. See, e.g., Benanti et al., Organic solar cells: An overview focusing on active layer morphology, Photosynthesis Research, vol. 87, pp. 73-81 (2006); and Kippelen et al., Organic Photovoltaics, Energy Environ. Sci., vol. 2, no. 3, pp. 251-261 (2009).
The field of organic solar cells has benefited from the development of light-emitting diodes based on similar technologies, which have entered the market recently. For a review of the field of organic solar cells, discussion of their different production technologies, and discussion of parameters to improve their performance, see Hoppe et al., Organic solar cells: An overview, Journal of Materials Research, Vol. 19, Issue 07, pp 1924-1945 (2004).
Among the several challenges to improve the performance of organic photovoltaics (OPVs) is the Shockley-Queisser limit (˜33.7%), as defined above. Thermodynamic modeling predicts that using materials capable of multiple exciton generation (MEG) in a single junction solar cell could theoretically circumvent the Shockley-Queisser limit and increase the upper limit of power conversion efficiency from 33.7% to 44%. The assumption is that SF results in forming two triplet excitons, each of which produces an electron-hole pair.
Recently it was reported that the organic dye pentacene could be useful in providing greater solar efficiency. Congreve et al., External Quantum Efficiency Above 100% in a Singlet-Exciton-Fission-Based Organic Photovoltaic Cell, Science, vol. 340, no. 6130, pp. 334-337 (2013). Pentacene is a polycyclic aromatic hydrocarbon consisting of five linearly-fused benzene rings, which acts as an organic semiconductor. As reported by Congreve, a photovoltaic cell based on pentacene could generate two electrons from a single photon, i.e., more electrical current from the same amount of sun light. Various approaches have been taken in efforts to design compounds that will produce more efficient singlet fission. See J. C. Johnson et al., Toward Designed Singlet Fission: Solution Photophysics of Two Indirectly Coupled Covalent Dimers of 1,3-Diphenylisobenzofuran, J. Phys. Chem. B, 117, 4680 (2013).
Singlet-exciton fission describes the process in which an arriving photon generates two “excitons” (excited states) that can be made to yield two electrons. Singlet exciton fission is a spin-allowed process for generating two triplet excitons from a single absorbed photon. Fission of singlet excitons into two triplet exciton pairs is spin conserving and, therefore, spin allowed. Theoretically, the efficiency of a conventional solar cell could be improved if a molecular material capable of singlet fission could be incorporated.
The production of two triplet excitons from the absorption of a single photon. To implement this, the two triplets from the singlet fission material need to be successfully harvested. Singlet fission (SF) could dramatically increase the efficiency of organic solar cells by producing two triplet excitons from each absorbed photon. While this process is known, most descriptions have assumed the necessity of a charge-transfer intermediate. See Zimmerman et al., Mechanism for Singlet Fission in Pentacene and Tetracene: From Single Exciton to Two Triplets, J. Am. Chem. Soc., 133 (49), pp. 19944-19952 (2011). For an in depth discussion of singlet fission, see Smith et al., Singlet Fission, Chem. Rev., 110, pp. 6891-6936 (2010).
Although several existing materials exhibit singlet fission, these materials are generally based on aggregates of conjugated and/ or aromatic molecules, including, for example, acenes, polyenes, and caratenoids. Singlet fission has also been previously demonstrated in polymers including poly-thiopehenevinylene and poly-pheneylenevinylene. Thiophene dioxide (TDO)-containing systems have been studied for other applications. However, these studies were predominately focused on the basic science of molecular TDO-containing entities, particularly on light emission. These previously studied molecular singlet fission systems may offer good singlet fission efficiency; however, they are not very adjustable, efficiencies are low, and triplet lifetimes are very short. The combined effects of which make applications of existing molecular singlet fission systems limited and applications of existing polymeric singlet fission systems impractical.
Conjugated polymers that have been suggested in the literature for use in organic photovoltaic devices (“OPV devices”) do still suffer from certain drawbacks. For example, many polymers suffer from limited solubility in commonly used organic solvents, which can inhibit their suitability for device manufacturing methods based on solution processing, or show only limited power conversion efficiency in OPV bulk-hetero-junction devices, or have only limited charge carrier mobility, or are difficult to synthesize and require synthesis methods which are unsuitable for mass production.
While SF has been extensively studied in molecular crystals and aggregates, made possible by an intermolecular process, intramolecular SF is not well understood in molecular and polymeric materials. Fundamentally, a core challenge is the modular synthetic design of building blocks for molecules and polymers that can undergo SF. Coupling chemical structure design with the mechanistic understanding of the physical processes of multiple exciton generation (“MEG”) could open avenues of exploration in parallel using families of materials, rather than the current serial approach targeting single compounds, which are generally based on acenes, oligoenes, and select polymeric materials.
Intramolecular SF is a process that has been rarely invoked in soft materials—it has only been observed in oligoenes (carotenoids) and polyenes (polydiacetylene), as well as a thiophene-containing conjugated polymer. It is postulated that these materials are capable of producing multi-exciton states through charge delocalization across these large molecules. However, such observation does not provide the necessary guidelines to build new materials. There is a need for designing and synthesizing novel materials for intramolecular singlet exciton fission in small molecules and polymers that are efficient and configurable singlet fission materials, which important for developing low cost, efficient organic (or hybrid) photovoltaic technologies. Also, there is a need for developing solution processable small molecule and polymeric singlet fission materials which allow for effortless device assembly through a variety of low-cost processing techniques, where these materials may additionally have applications in fuel cells.
The vast efforts towards developing efficient solar cells based on organic materials1,2 have led to advancements in processing and characterizing semiconducting molecules and polymers3-5, as well as engineering organic photovoltaic (OPV) device architectures that have yielded significant increases in efficiency6-9. To raise the theoretical limit of power conversion efficiency above the Shockley-Queisser limit10,11, organic materials capable of generating multiple excitons from a single photon have been explored in devices12,13, with reported external quantum efficiencies exceeding 100%14. In these systems, the primary multiexciton generation mechanism is intermolecular singlet fission (xSF) within molecular aggregates or crystals15, wherein the absorption of one photon leads to the formation of two triplet excitons on adjacent molecules16. Because of the intermolecular nature of this process, strong electronic coupling between nearest neighbors is required and, as such, the efficiency of this process is highly sensitive to the crystallinity of the film17 and the presence of functional groups that expand the unit cell18. A more widely applicable route to functional devices would preferably be based on intramolecular processes, where the fission efficiency is an intrinsic property of the designed material—that is, it is not dependent on molecular orientation, intermolecular coupling, or long-range order, among other constraints. Furthermore, this enhances the possibility of using polymers as fission materials that have tunable chemical structure to control solution processability, film morphology, and various other physical and electronic properties. However, intramolecular singlet fission (iSF) has not been observed with high yield and, as a result, the mechanism is poorly understood. iSF has been observed in a few systems (including several vinylene-containing polymers)19-21, but fission in these systems is typically an activated process requiring a photon with energy in excess of the bandgap. Only the oligoenes (such as carotenoids) have demonstrated non-activated intramolecular singlet fission, although yields have not exceeded 30%22,23. The present understanding of iSF provides little insight into how multiple exciton generation can be modularly designed as a feature in molecular materials24.
There is still a need to better understand iSF, however, some general guidelines for efficient SF have been suggested from studies of intermolecular processes.
Recently, it was found that the coupling between the singlet state and the ME states is weak, but the MEG process is mediated by a strongly coupled intermediate CT state.
Therefore, there is still a need for singlet-fission capable organic semiconducting (“OSC”) materials that are easy to synthesize, especially by methods suitable for mass production, show good structural organization and film-forming properties, exhibit good electronic properties, especially a high charge carrier mobility, good processability, especially a high solubility in organic solvents, and high stability in air. Especially for use in OPV cells, there is a need for OSC materials having a low bandgap, which enable improved light harvesting by the photoactive layer and can lead to higher cell efficiencies, compared to the polymers discussed in the literature.